1. Field of the Invention
The present invention relates to well logging means and methods in general and, more particularly, to a well logging means and method for determining the density of a fluid in the borehole, formation density and borehole diameter using back-scattered gamma radiation.
2. The Prior Art
The prior art is best represented by U.S. Pat. No. 4,939,362 issued Jul. 3, 1990, and assigned to a common assignee. The invention described in this patent is a density meter in the form of a sonde adapted to be inserted into a borehole. The sonde includes a gamma ray source and a detector assembly for detecting the back-scattered gamma rays and provide a signal corresponding to the counted rays. Caliper means on the sonde continually measure the diameter of the borehole as the sonde passes therethrough and provides a caliper signal corresponding to the diameter. The count signals and caliper signals are sent to the surface for processing to generate an output signal corresponding to the density of the fluid in the borehole.
Supernaw et al recognized that the gamma rays are Compton scattered in the fluid and formation. The detector assemblies cannot determine where the scattering occurs and simply register the reception of a gamma ray. The count rate is determined by the number of these gamma rays observed during a specific time interval. If this count rate is used to calculate the density, the result will reflect a density that is a combination of the fluids in the borehole and the formation, and consequently be erroneous, due to the fact that the formation contribution is significant and dependent on the fluid densities, the borehole diameter, as well as the formation density. To obtain the correct density of the borehole fluids, it is necessary to account for the formation. Supernaw et al do not consider nor provide a means to remove or correct for the contribution of the formation.